In a beamformer arrangement, acoustic signals are processed in such a way, that signals from a preferred direction are enhanced with respect to signals from other directions. This characteristic is in particular useful for speech signal processing, for example, for hands-free telephone sets or speech recognition systems in vehicles.
For such systems, echo compensation is a basic issue. Disturbing echoes may result from signals which are, for example, output by a loudspeaker of the same system and detected together with the wanted signal by microphones of a microphone array in the beamformer arrangement. In the case of a hands-free telephone set, for instance, signals from a far end are output by a loudspeaker at the near end where they are again detected by the microphones. In order to avoid that these signals are transmitted back to the far end, echo compensation or echo cancellation has to be performed.
Several methods and systems for echo compensation are known (see, e.g. Acoustic Echo and Noise Control, E. Hañsler and G. Schmidt, John Wiley & Sons, New York, 2004).
One known method uses a separate acoustic echo compensator, for example an acoustic echo canceller, for each microphone signal. This method yields a very robust echo compensation but is computationally intensive. In particular, the computational costs increase with the number of microphones in a microphone array.
Alternatively, according to another method, only one acoustic echo compensator, for example an acoustic echo canceller, is used which operates after a beamformer, i.e. which operates on a beamformed signal. This method is computationally efficient as it requires only one acoustic echo compensator independent of the number of microphones in the microphone array.
However, when using only one acoustic echo compensator operating on the beamformed signal, residual echo components in the echo compensated signal can be observed when the beamformer changes from one steering direction to another. For example, in a vehicle, the beamformer may provide a steering direction corresponding to the position of a first speaker. If an utterance from a second speaker, at a different position, should be processed, the beamformer needs to provide a second steering direction according to the position of the second speaker.
After the change-over from the first steering direction to the second steering direction, the signal quality of the echo compensated beamformed signal can be impaired by the presence of a residual echo signal until the acoustic echo compensator have re-adapted for the second steering direction. This, however, may take some time.
FIG. 9 depicts a prior art system comprising a beamformer and a plurality of acoustic echo cancellers.
In particular, a microphone array comprising a plurality of microphones 901 is shown. A microphone 901 may detect signals from a loudspeaker 902 and/or from wanted sound sources, in particular, speakers 903. A steering direction 904 of the beamformer arrangement may be provided corresponding to the direction of arrival of a signal originating from one of the speakers 903. Speaker localization module 905 allow to determine the position of the speaker 903 and/or the direction of arrival of a wanted signal originating from the speaker 903. A beamformer 906 may be used to perform time delay compensation of the microphone signals. The beamformer may further process the time delay compensated microphone signals yielding a beamformed signal.
Before beamforming, acoustic echo cancellers 941 are used to perform echo compensation of each microphone signal individually. The beamformed and echo compensated signal may be provided to a hands-free system 910.
If echo compensation is performed before beamforming, for each microphone signal a separate acoustic echo canceller 941 needs to be used. Such a system, as for example shown in FIG. 9, has the advantage that if the beamformer 906 changes from one steering direction 904 to another, the acoustic echo cancellers 941 do not need to be re-adapted. In other words, the beamformer 906 uses echo compensated microphone signals. Therefore, a change from one steering direction 904 of the beamformer 906 to another has no influence on echo compensation.
A drawback of such a system as shown in FIG. 9, is, however, that using an individual acoustic echo canceller 941 for each microphone signal is computationally intensive, wherein the computational costs increase with the number of microphones 901 in the microphone array.